DS R4 and DS Firmware?

Hello DS modders. I had my DS Lite for a year and a half now and I have finally urged myself to mod my DS. However after a couple of days of researching, I heard of something called R4. This is a slot one flash card right? I saw an excellent youtube video in using the R4 with the micro sd and the packaging of it. I'm still clueless if this R4 works on all DSs (this is my main concern) . Is there a certain firmware my DS must be on? Is a 17ish year DS enough? How much does an R4 cost? Finally, what is the best website to buy a R4. I will appreciate any clarifications and help, thank you.

uh, you cant update the ds's firmware but they have this bundle for r4's that's like $50 but it includes a lot of stuff and you can put a whole bunch of stuff on it like movies,music,pictures, and a lot of games. if you want to play ds games on the r4(which i prefer) its really easy and takes up very little space. the website i recomend for the ds games is ndsroms.com